I know what you mean...
You're not even listening, but you're still trying to act like you understand everything they said.
They tell you about their bad day. You say, 'I know what you mean,' even though you were playing video games the whole time.
They explain a complicated plan. You say, 'I know what you mean,' when you're just thinking about snacks.
They talk about their weird dream. You say, 'I know what you mean,' even though you're just trying to get to sleep.